Hereinafter, a conventional electronic part will be described.
In the prior art, a surface acoustic-wave device (hereinafter, referred to as the “SAW device”) is described as an example of an electronic part.
In recent years, a small and light SAW device is prevalently used for electronic equipment, such as various types of mobile communication terminal equipment. Especially, in a radio circuit section of a mobile-phone system within a band of 800 MHz to 2 GHz, an SAW filter has been widely used. Such an SAW filter is formed by a lithium-tantalate (hereinafter, referred to as the “LT”) substrate which is cut out of a Y-sheet at a cutout angle of 36° as its rotational angle around the X-axis in the Z-axis direction, or a so-called 36° Y-cut X-propagation LT (hereinafter, referred to as the “36° YLT”) substrate. However, depending upon a place at which a filter is used in a mobile-phone system or its radio circuit section, filter characteristics are required, such as a small insertion loss within a further passing band and a steep skirt property of the filter, as well as a high suppression level within a block band. In order to meet such demands, there is a method in which an LT substrate is used that is cut out of a Y-sheet at a cutout angle of 42° as its rotational angle around the X-axis in the Z-axis direction, or a so-called 42° Y-cut X-propagation LT (hereinafter, referred to as the “42° YLT”) substrate is used. According to this method, an SAW filter can be realized which has a smaller loss and a steeper filter skirt property than those of the conventional 36° YLT substrate. Such a method is described in Japanese Patent Laid-Open No. 9-167936 specification.
However, similarly to the conventional 36° YLT substrate, such a 42° YLT substrate has a great thermal-expansion coefficient in the direction where a surface acoustic wave is propagated. Beside, the elastic constant itself varies according to the temperature. Thereby, the frequency characteristics of a filter may also be largely shifted by approximately −35 ppm/° K according to a change in the temperature. This is disadvantageous in temperature characteristics. For example, taking a PCS transmission filter from the United States into account, its center frequency of 1.88 GHz at the room temperature changes by about ±3.3 MHz or some 6.6 MHz within an range of ±50 ° C. In the case of the PCS, there is an interval of only 20 MHz between its transmission band and reception band. Hence, also considering the dispersion of frequencies in its production, in practice, the transmission and reception interval is only substantially 10 MHz for the filter. Thus, for example, if the transmission band is desired to be secured over the entire temperature (i.e., within the whole range of ±50° C at the room temperature) an adequate attenuation value cannot be obtained on the reception side.